Devices that require high quality epitaxial indium phosphide thin films traditionally are formed on indium phosphide substrates. These substrates, however, are characterized by small wafer size, brittleness and high cost. Accordingly, there is much interest in developing techniques for growing high quality epitaxial indium phosphide thin films on other material systems, for example, silicon substrates, which offer superior mechanical and cost performance relative to indium phosphide substrates.
A wide variety of different approaches for forming indium phosphide thin films on silicon substrates have been proposed. For example, indium phosphide layers have been formed directly on a silicon substrate. In accordance with these processes, an indium phosphide layer may be formed in one or two steps. In a conventional one-step process, a single thin film of monocrystalline indium phosphide is formed directly on silicon substrates. In a conventional two-step process, an amorphous indium phosphide buffer layer initially is formed on a silicon substrate and a final thin film of monocrystalline indium phosphide is formed on the amorphous indium phosphide buffer layer. The indium phosphide layers formed by these methods, however, are characterized by high defect rates and low quality surface morphology and, in general, are not suitable for large scale semiconductor device fabrication. The relatively large lattice mismatch (on the order of 8%) between indium phosphide and silicon is a major contributor to the low quality of the indium phosphide films formed by these methods.
In an effort to reduce lattice-mismatch induced defects and surface roughness in indium phosphide thin films formed on silicon substrates, U.S. Pat. No. 4,965,224 has proposed forming a gallium arsenide buffer layer on a silicon substrate and an amorphous indium phosphide buffer between a monocrystalline indium phosphide thin film and the gallium arsenide buffer layer. Because gallium arsenide has a lattice constant that is intermediate between the lattice constants of the indium phosphide thin film and the silicon substrate, U.S. Pat. No. 4,965,224 asserts that the distortion in the final monocrystalline indium phosphide thin film is reduced relative to the indium phosphide thin films formed by the methods described in the previous paragraph. The lattice mismatch (on the order of 3.8%) between indium phosphide and gallium arsenide, however, is relatively high. In addition, growth of indium phosphide on amorphous indium phosphide produces a defect rate and a surface morphology that, in general, is not suitable for large scale semiconductor device fabrication.